A rasp is usually a woodworking file. Unlike a metal file, where the teeth are set very close together, a rasp has teeth set quite wide apart as well as being deeper. This leads to a much coarser cut and allows for quite a lot of surface material to be removed in one stroke.
Rasps are also used to remove surfaces of other materials such as horses hooves and they are often used to shape soft stone when carving or sculpting.
Rasps are designed to shape a surface quickly and can be bought with a variety of tooth settings so the finish can get smoother as the work goes on. Much the same as using a heavy grit sandpaper before moving up to a finer grit for finishing.
Rasps, as with metal files, are designed to cut or plane on the stroke away from the body. Using a rasp or file in both directions will eventually ruin the teeth. Rasps, again, as with files, can be bought in a few shapes.
Rasps can be flat on both sides, flat on one side and half-round on the other, completely round or triangular. These shapes allow for most types of shaping to be done with a rasp before the work is finished using finer tools.
See also our project on using files.